An active-matrix type display apparatus such as an LCD (liquid crystal display) and an organic EL (Electro Luminescence) display apparatus is widely used in respect of display performance, energy saving, or the like. In particular, the active-matrix type display apparatus has almost come to constitute the mainstream of a display for cellular phones, PDAs (personal digital assistance), PCs, lap-top PCs, TVs or the like. In these display apparatuses, a TFT (field-effect thin film transistor) substrate is normally used
For example, a liquid crystal display has a configuration in which a display material such as a liquid crystal is filled between a TFT substrate and an opposing substrate, and a voltage is selectively applied to this display material for each pixel. Here, a TFT substrate means a substrate in which a TFT using, as an active layer, a semiconductor thin film (also referred to as a semiconductor film) such as an amorphous silicon thin film and a polycrystalline silicon thin film is arranged. The above-mentioned image display apparatus is driven by the active matrix circuit of a TFT. Since TFTs are arranged in the shape of an array, a TFT substrate is also referred to as a “TFT array substrate”.
Meanwhile, in a TFT substrate used for a liquid crystal display or the like, a set of a TFT and one pixel of the screen of a liquid crystal display (this set is referred to as one unit) are arranged longitudinally and laterally on a glass substrate. In a TFT substrate, a gate wire is arranged longitudinally at equal intervals on a glass substrate, and a source wire or a drain wire are arranged laterally at equal intervals. In addition, a gate electrode, a source electrode, and a drain electrode are respectively formed in the above-mentioned unit which constitutes each pixel.
A transistor using the above-mentioned silicon thin film is produced by using a silane-based gas, and hence, it is disadvantageous in respect of safety or equipment cost. In addition, an amorphous silicon thin film has an electron mobility which is as low as about 0.5 cm2/Vs when used in a TFT. In addition, since an amorphous silicon thin film has a small band gap, it may absorb visible rays to cause malfunction. Moreover, a polycrystalline silicon thin film requires a heating process which is conducted at relatively high temperatures, thereby incurring a large amount of energy cost. In addition, this film is hard to be formed directly on a large-sized glass substrate.
Under such circumstances, a TFT using an oxide semiconductor thin film which can be formed at low temperatures has been actively developed. In addition, with the development of the above-mentioned TFT, development of a semiconductor device using an oxide semiconductor thin film has been actively developed. Further, in order to obtain a flat panel display (Flat Panel Display: FPD) which has a reduced thickness, is light in weight and is resistant to fracture, an attempt has been made to use instead of a glass substrate a resin substrate which is light in weight and flexible.
For example, Patent Document 1 discloses a technology of a thin film transistor using an oxide semiconductor film composed mainly of zinc oxide which is capable of being formed into a film at low temperatures.
Patent Document 2 discloses a technology of a field-effect transistor in which an amorphous oxide containing at least one of In, Zn and Sn is used as an active layer (channel layer), and a gate electrode, a source electrode and a drain electrode.
Patent Document 3 discloses a technology of a transparent conductor material containing an amorphous oxide shown by a general formula ZnxMyInzO(X+3Y/2+3Z/2) (wherein M is aluminum or gallium).
Patent Document 4 discloses a technology of a method for producing a field-effect transistor including a step of forming an active layer containing an amorphous oxide in an atmosphere which contains at least one selected from ozone gas, a nitrogen oxide gas, oxygen-containing radicals, atomic oxygen, oxygen ions and oxygen radicals.
Patent Document 1: JP-A-2003-298062
Patent Document 2: JP-A-2006-165527
Patent Document 3: JP-A-2000-044236
Patent Document 4: JP-A-2006-165531